Following multiple infections with N. dubious larvae, eight strains of inbred mice showed a wide variation in their ability to develop immunity to oral challenge with this parasite. Only two of the eight strains developed the self-cure reaction. In the S-W strain, (1) male and female maintained a striking degree of immunity for 50 weeks; (2) neither age or sex resistance was evident after a single oral infection with 50 N. dubius larvae; (3) mice would develop immunity to N. dubius if previously exposed to the parasite as neonates; and (4) it appears that functional immunological maturity in this host-parasite system develops between 3 and 4 weeks of age.